1. Technical Field
The present invention relates to a black ink composition containing a pigment.
2. Related Art
An ink jet recording method is a printing process for conducting printing by letting ink droplets fly and adhere to a recording medium such as paper. Inks used in the method are usually prepared by dissolving various kinds of water-soluble dyes in water or in mixtures of water and water-soluble organic solvents. It is pointed out that images formed with such inks containing water-soluble dyes are usually inferior in water resistance and light resistance.
On the other hand, inks prepared by dispersing pigments in aqueous solvents are superior in water resistance and light resistance. For example, aqueous pigment inks in which pigments are dispersed with surfactants or polymer dispersants have been proposed. However, in some these inks, an increase in colorant content for increasing the printing density of a recorded matter causes a drastic increase in ink viscosity.
In order to solve this problem, proposed is a dispersion of a self-dispersing pigment, which can be dispersed in an aqueous solvent by itself without requiring any use of dispersant such as a surfactant or a polymer dispersant, due to surface-active hydrogen or its salt introduced to the pigment surface in a certain amount or more.
In JP-A-10-237349, JP-A-8-3498, and WO01/94476, it is described that the so-called self-dispersing pigment, which is dispersible in the absence of dispersants as described above, has characteristics, for example, that it increases the optical density (OD) value of an image when used as a colorant in an ink.
Furthermore, as an ink set having an excellent color-developing property, JP-A-2009-256605 discloses an ink set including a black ink composition containing a self-dispersing pigment having a hydrophilic group on its surface.
However, images formed by these inks containing the self-dispersing pigments as colorants are usually inferior in fixing property and may get dirty when they are rubbed with, for example, a finger, a line marker, or paper. Thus, recorded matters of these images may have insufficient abrasion resistance. Accordingly, in order to improve the fixing properties of self-dispersing pigments to recording media, an ink composition containing a resin that shows adhesion to recording media has been proposed.
JP-A-2001-81366 and JP-A-2002-80761 propose, for example, a recording solution including a colorant, resin particles, and water; and a recording solution for ink printers including resin particles and an aqueous dispersion of a pigment, as ink compositions containing resin particles.
Furthermore, JP-A-2009-256606 discloses an ink set including a black ink composition that contains a self-dispersing pigment having a hydrophilic group on its surface and a color ink composition that contains a self-dispersing pigment having a hydrophilic group on its surface via a phenyl group. Each ink composition contains a resin emulsion.
Nevertheless, there is still a demand for an ink composition that can realize an image in which both enhancement of a color-developing property and satisfactory inhibition of a time-dependent change in printing density due to pigment sedimentation are achieved.
JP-A-2006-104452 discloses an actinic ray-curable ink-jet ink containing particles having an average diameter of 0.08 to 0.25 μm in which the number of coarse particles having a particle size not smaller than 1 μm is less than 6.0×105/μL.